The invention relates generally to ultrasound systems and more specifically to a reconfigurable array of multi-level transmitters. One specific application for such an array is in medical diagnostic ultrasound imaging systems. Another specific example is for non-destructive evaluation of materials, such as castings, forgings, or pipelines.
An ultrasound imaging system forms an image by acquiring individual ultrasound lines (or beams). The lines are adjacent to each other and cover the target area to be imaged. Each line is formed by transmitting an ultrasonic pulse in a particular spatial direction and receiving the reflected echoes from that direction. The spatial characteristics of the transmitted wave and the characteristics of the receive sensitivity determine the quality of the ultrasound image. It is desirable that the ultrasound line gathers target information only from the intended direction and ignores targets at other directions.
Conventional ultrasound imaging systems comprise an array of ultrasonic transducer elements that are used to transmit an ultrasound beam and then receive the reflected beam from the object being studied. Such scanning comprises a series of measurements in which the focused ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received, beamformed and processed for display. Typically, transmission and reception are focused in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver may be dynamically focused at a succession of ranges along the scan line as the reflected ultrasonic waves are received.
For ultrasound imaging, the array typically has a multiplicity of transducer elements arranged in one or more rows and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements in a given row can be controlled to produce ultrasonic waves that combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused in a selected zone along the beam.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal zone in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer element. The time delays are adjusted with increasing depth of the returned signal to provide dynamic focusing on receive.
The quality or resolution of the image formed is partly a function of the number of transducer elements that respectively constitute the transmit and receive apertures of the transducer array. Accordingly, to achieve high image quality, a large number of transducer elements is desirable for both two- and three-dimensional imaging applications. The ultrasonic transducer elements are typically located in a hand-held transducer probe that is connected by a flexible cable to an electronics unit that processes the transducer signals and generates ultrasound images. The transducer probe may carry both ultrasound transmit circuitry and ultrasound receive circuitry.
Conventional medical ultrasound imaging creates two-dimensional, cross-sectional images using one-dimensional linear or phased array transducers. These transducers are built with approximately 100 to 200 elements arranged in a linear fashion. The transducer elements are connected to high-voltage transmitters or pulsers in the system. The transmitters or pulsers send waveforms to the transducer elements, which in turn convert the electrical waveforms into acoustic waves. By properly controlling the waveforms, a focused sound beam is generated. The signal level of the electrical waveforms can be several hundred volts in order to generate the desired level of acoustic energy. Connecting a few hundred transducer elements to the system is technically feasible with current technology. Current ultrasound systems address the problem of increased channel count by attempting to integrate discrete electronics at the board level. These systems typically are able to drive only about 128-256 channels and consume a large amount of power. Most of this power is expended to drive the cable.
Two-dimensional transducer arrays are required for electronically steered three-dimensional imaging. These types of transducer arrays typically employ several thousand elements. For proper beamforming, each one of these elements must be connected to a beamforming channel. Connecting several thousand elements to respective pulsers in the system is technically not feasible because a cable bundle of coaxial or other wire comprising a sufficient number of conductors for several thousand elements would be too thick and heavy to be ergonomically viable. Also, a cable that would connect the system pulsers to the transducer elements would present a very large capacitance load compared to the impedance of the two-dimensional array element. Therefore, a majority of the pulser current would be drawn into the cable capacitance while only a small fraction of the current would be drawn into the transducer element. As a result, only a small fraction of the energy supplied by the pulser would be converted to acoustic waves. Consequently, for a large array of tiny elements, much more power would have to be supplied by the pulser circuitry than would be required from a linear array. This additional power requirement might be tolerable for a full-size clinical ultrasound scanner. However, it would be prohibitive for a portable system, which would not be able to supply sufficient cooling for the pulsers. In addition, the portable system would suffer drastically reduced battery life.
U.S. patent application Ser. No. 10/697,518, filed on Oct. 30, 2003, discloses the concept of integrating pulsers or transmitters directly in the probe handle. This solves the problem of power consumption due to the cable, but does not address the more pragmatic concerns about the amount of power expended by the actual pulser control architecture. In addition, this patent application does not address the actual architecture of the pulser control circuit and does not treat the transmit/receive circuit.
Further, to provide accurate imaging, bipolar transmitters are often used to produce the ultrasound pulses in the system. In contrast to unipolar transmitters, these transmitters typically generate waveforms defined by a sequence of square wave pulses of alternating negative and positive voltages. Advantageously, bipolar transmitters are inexpensive to make and easy to control, thereby making them a convenient choice over unipolar transmitters. However, bipolar transmitters provide a very limited voltage spectrum. In many systems, a larger number of voltage levels may be desirable to produce pulse sequences approximating signal waveforms, such as sinusoidal waveforms. Generating multiple voltage levels is generally expensive and difficult to implement. Further, transmitters capable of outputing numerous voltage levels are often inefficient and consume large amounts of power.
Accordingly, there is a need to solve the problem of driving a large number of small ultrasound transducers in a two-dimensional array configuration with minimal power expenditure and in a small footprint, wherein the transmitter is capable of producing multiple voltage levels.
Embodiments of the present invention may be directed to one or more of the challenges described above.